Performance comparisons of concatenated codes with iterative decoding for DS-CDMA systems with application to IS-95-based cellular systems

The remarkable performance of turbo codes for low SNRs in AWGN channels has encouraged the application of turbo codes to systems operating in different kind of channels. In this paper we consider an IS-95-based CDMA system operating in a multipath Rayleigh fading channel. We replace the convolutional encoder by a parallel and a serial turbo encoding scheme, and by a serial concatenation of an outer convolutional and an inner parallel turbo code, and instead of the Viterbi decoder we use iterative decoding based on the MAP algorithm. We perform BER and FER comparisons by varying system parameters such as the constraint length of the component convolutional codes, the interleaver lengths, and the wireless channel fading rate. The results obtained show that the concatenated schemes outperform the more complex convolutional scheme even for relatively short interleaving lengths, especially when FER is the desired figure of merit.